182 BACTERIOLOGY. 



Three gelatin tubes are inoculated in the usual manner 

 (p. 174) with the material to be plated. It is advisable to 

 select very wide tubes. The cotton projecting- from the 

 tube is then cut off, and the neck of the tube is rotated 

 rapidly in a flame in order to insure sterilization of the 

 outer layer of cotton, since this may subsequently, come 

 into contact with the gelatin. The cotton plug is then 

 pushed in a trifle, and the tube is closed by means of a rub- 

 ber cap. A boiled cork or rubber stopper may be used for 

 this purpose. It is then immersed in cold, or better, in ice- 

 water, and is rotated slowly in an almost horizontal posi- 

 tion. The gelatin should solidify in a thin, even layer, 

 over the inner wall of the tube. During the process of roll- 

 ing, care should be taken to prevent the gelatin from com- 

 ing into contact with the cotton plug. It will -not only 

 cause the cotton to adhere to the glass, but may prevent 

 access of air, and thus bring about partial anaerobic condi- 

 tions within the tube. 



A better method of rolling the tube is to do this on a 

 block of ice. A very slightly inclined groove can be made 

 in the ice by means of a test-tube filled with hot water. In 

 this case it is unnecessary to heat the cotton, or to use a 

 rubber cap or a stopper. The roll-tubes should be set aside 

 in a cool place (15°), to prevent collapse of the gelatin. 

 The colonies are then examined under the microscope and 

 transplantations can be readily made from these, especially 

 if the tube is wide. For transplanting, a platinum wire 

 should be used, the end of which is bent at right angles. 



Laboratory work.— The student will make Esmarch roll tubes of Yel- 

 low sarcine, and Petri dish cultivations of the Orange sarcine. 



Modified Potato Cultures. 



The method of making potato cultures, as described on 

 p. 167, is open to many of the same objections which were 

 made in connection with the ordinary gelatin plate method. 



